Inclusion Criteria:

In order to be eligible to participate in this study, an individual must meet all of the following criteria:

• Patients diagnosed with atrial fibrillation based on electrocardiographic documentation.
• Male or female greater than or equal to the age of 60 years.
• Have a CHA2DS2-VASc score ≥2.
• Have never been treated with Apixaban, or prior treatment with Apixaban <1 month.
• Provision of signed and dated informed consent form.
• Stated willingness to comply with all study procedures and availability for the duration of the study.
• Ability to take oral medication and be willing to adhere to the study intervention regimen.

Exclusion Criteria:

An individual who meets any of the following criteria will be excluded from participation in this study:

• Patients who are not a candidate for oral anticoagulation as assessed by a treating physician including the presence of active bleeding.
• Prior treatment with Apixaban for >1 month. (Prior exposure to warfarin or novel direct oral anticoagulants other than Apixaban is not an exclusion criterion).
• Known hypersensitivity to warfarin or Apixaban.
• CHA2DS2-VASc score <2.
• Valvular AF defined as history of rheumatic valve disease, moderate or greater mitral stenosis, and presence of a mechanical cardiac valve.
• Need for dual anti-platelet therapy with aspirin and another agent such as a thienopyridine.
• Aspirin monotherapy with doses > 100 mg if the patient is unable to reduce the dose to <100 mg.
• Severe renal insufficiency (serum creatinine level of >2.5 mg/dl or calculated creatinine clearance of <25 ml/minute) or dialysis.
• Prior severe bleeding including intracranial hemorrhage and GI bleed requiring transfusion.
• Recent stroke (within 7 days).
• Known diagnosis of dementia or dementia diagnosed at first evaluation.
• Presence of MRI non-compatible implanted devices including cardiac implantable electronic devices. Patients with MRI conditioned cardiac implantable electronic devices will also be excluded. This is done in light of concerns regarding patient safety in MRI scanner.
• Inability to undergo MRI due to claustrophobia.
• Current or expected systemic treatment with strong dual inducers of CYP3A4 and P-gp (e.g., rifampin, carbamazepine, phenytoin, St. John’s wort) because such drugs will decrease exposure to apixaban.
• Current or expected systemic treatment with strong dual inhibitors of CYP3A4 and P-glycoprotein (e.g., ketoconazole, itraconazole, ritonavir, or clarithromycin) in patients who would qualify for Apixaban dose of 2.5 mg BID.
• Currently enrolled in another investigational device or drug trial that has not completed the primary endpoint or that clinically interferes with the current study endpoints.
• Co-morbid condition(s) that could limit the subject's ability to participate in the trial or to comply with follow-up requirements, or that could impact the scientific integrity of the trial.
• Prisoners or subjects who are involuntarily incarcerated. Subjects who are compulsorily detained for treatment of either a psychiatric or physical (e.g., infectious disease) illness.
• Severe comorbid condition with life expectancy ≤1 year.
• Active alcohol or drug abuse or psychosocial reasons that make study participation impractical.